Tidal power
Tidal power has been used for centuries in the form of tidal mills, although not in the Exmoor area. The Bristol Channel, however, has the second highest tidal range in the world and is a prime site for the development of tidal power. A Severn barrage has been investigated for many years, the first suggestion being in 1840 and the first major plans being in 1933. A series of proposals put forward in the 1980s suggested that a barrage was feasible from anywhere east of Porlock and plans were drawn for one from Minehead. These were dropped on cost grounds but there is renewed interest to meet the UK's targets for renewable energy. A modern barrage could generate 6% of the energy needs of England and Wales, with a 8640MW capacity. It would avoid the emission of 18 million tonnes of carbon dioxide each year.
There are few suitable sites for tidal barrages. The UK has 6 out of the 20 most suitable sites in the world with the Severn ranking amongst those with the highest potential electricity output.
Advantages:
T idal power produces no greenhouse gases or other waste. It needs no fuel, although other sources of electricity may be used to make a barrage part of a pumped storage scheme. It produces electricity reliably. Barrages are not expensive to maintain. Tides are totally predictable. Offshore turbines and vertical-axis turbines are not ruinously expensive to build and do not have a large environmental impact. A Severn barrage would reduce the risk of flooding in the Severn Estuary, which, through climate change, is is thought to increase by a hundredfold by 2075, at which time flooding in the area would cost £4 billion annually.
Disadvantages
A barrage across an estuary is very expensive to build, and affects a very wide area - the environment is changed for many miles upstream and downstream. Many birds rely on the tide uncovering the mud flats so that they can feed. Tide only provides power for around 10 hours each day, when it is moving in or out.
Tidal turbine
The only active use of tidal power in the Exmoor area until recent times has been the lock gates to the harbour at Porlock Weir. These can be closed at high tide to hold back water, which is then released rapidly at low tide to scour the build up of pebbles from the entrance to the harbour. Across the channel in Swansea Bay, a feasibility scheme has been drawn up for the creation of tidal lagoons for the generation of tidal power using similar principles. Exmoor, however, has become site for the world's first freestanding tidal turbine electricity generator.
Tidal generator off Foreland Point
The £3 million scheme at Lynmouth is an experiment. The tidal currents of the Foreland are very strong and in spring 2003 an 80 tonne steel pile was pushed into the seabed off the Point. A single turbine and rig was then fixed to the pile. This was arranged so that the blade – similar to that on a wind generator – could be raised on the rig for servicing. The 11metre long blade can generate 300 kilowatts of electricity. At present this is a small generator, not connected to the National Grid, and the electricity is just used to create waste heat. The turbine has been working very efficiently and it is hoped soon to add another blade to make use of both ebb and flow tides. The development company is hoping that the full scale production generators will be twice the size, with twin blades, each capable of producing 1000 kilowatts of power. The generators will be four times more efficient than wind generators of the same size, cheaper than offshore wind turbines and less obtrusive, as the blades are normally out of sight. Not only that but the power source is more predictable and reliable. It is thought that a 'farm' of ten such generators in that location would produce enough power for the whole Exmoor area.
Blade viewed from gantry (courtesy of Marine Current Turbines)
Blade raised for servicing (courtesy of Marine Current Turbines)
It is not currently known whether the next stage in the project will continue off Exmoor. The development company has permission to bring a cable onshore to connect with the National Grid. The National Park Authority is not otherwise involved as its planning powers do not extend below the high tide limit. The visual impact of the generators could be eliminated by a design in which the rig is normally completely submerged but can be extended to raise the rotor above water level for maintenance.